On the insensitivity of single field planar dosimetry to IMRT inaccuracies
On the insensitivity of single field planar dosimetry to IMRT inaccuracies
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DOI：
10.1118/1.3425781
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发表时间：
2010-06-01
期刊：
影响因子：
3.8
通讯作者：
Kruse, Jon J.
中科院分区：
文献类型：
--
作者：
Kruse, Jon J.
目的：报告单场平面测量的灵敏度，在确定调强放射治疗计划计算精度差。方法：三个调强放射治疗计划头颈部癌症进行了广泛的质量保证。在圆柱体模上重新计算计划，并在每个计划中使用离子室测量8到18个低梯度点。这些计划中测量的每个点均在计划系统预测剂量的4%范围内，并且这些计划被判定为可用于临床。每个计划，然后重新优化积极的剂量限制，使新的治疗领域更高度调制比原来的计划。非常复杂的领域可以计算准确度较低，这些计划在圆柱体模的离子室测量证实了显着的剂量误差-在每个计划中的几个测量点的计算剂量的差异超过4%，最大的单一偏差为10.6%。这三个计划被判定为临床使用不可接受。然后用两种单独场平面剂量测定方法分析所有六个计划（三个可接受，三个不可接受）：使用电子射野成像设备（EPID）和离子室阵列进行射野成像。对每组平面测量值进行伽马分析，采用2%/2 mm的一致距离（DTA）和3%/3 mm的DTA标准，以确定伽马分析阈值，该阈值将区分有缺陷的计划和可接受的计划。采用EPID和2%/2 mm DTA标准，可接受的IMRT计划中88.2%至92.8%的像素通过伽马分析，对于不可接受的调强放射治疗计划，通过率在87.5%~ 91.9%之间。采用离子室阵列和2%/2 mm DTA标准，可接受计划中92.4%至94.9%的点通过伽马分析，而不可接受计划中86.8%至98.3%的点通过伽马分析。当伽马标准扩展到3%/3 mm DTA时，可接受和不可接受计划之间的差异进一步减小。通过伽马分析的像素的一部分被发现是一个贫穷的预测与平面剂量计的剂量测定精度，以及两套伽马criteris.Conclusions：解构的调强放射治疗计划的字段由字段QA需要复杂的分析方法，如伽马函数。伽马函数的一个组成部分--一致距离在复合计划中具有临床相关性，但当应用于个体高度调制射野时，它可能掩盖重要的剂量测定误差。虽然单场平面剂量测定可能包括有效QA协议的一个方面，但单场测量的伽马分析对总体计划的重要剂量测定不准确性不敏感。(C)2010年美国医学物理学家协会。[DOI：10.1118/1.3425781]
Purpose: To report on the sensitivity of single field planar measurements in identifying IMRT plans with poor calculational accuracy.Methods: Three IMRT plans for head and neck cancer were subjected to extensive quality assurance. The plans were recalculated on a cylindrical phantom and between eight and 18 low gradient points were measured in each plan with an ion chamber. Every point measured in these plans agreed to within 4% of the dose predicted by the planning system and the plans were judged acceptable for clinical use. Each plan was then reoptimized with aggressive dose constraints so that the new treatment fields were more highly modulated than the ones from the original plans. Very complex fields can be calculated less accurately and ion chamber measurements of these plans in the cylindrical phantom confirmed significant dosimetric errors-Several of the measured points in each plan differed from the calculated dose by more than 4%, with a maximum single deviation of 10.6%. These three plans were judged unacceptable for clinical use. All six plans (three acceptable, three unacceptable) were then analyzed with two means of individual field planar dosimetry: Portal imaging with an electronic portal imaging device (EPID) and an ion chamber array. Gamma analysis was performed on each set of planar measurements with 2%/2 mm distance to agreement (DTA) and 3%/3 mm DTA criteria to try to determine a gamma analysis threshold which would differentiate the flawed plans from the acceptable ones.Results: With the EPID and 2%/2 mm DTA criteria, between 88.2% and 92.8% of pixels from the acceptable IMRT plans passed the gamma analysis, and between 87.5% and 91.9% passed for the unacceptable IMRT plans. With the ion chamber array and 2%/2 mm DTA criteria, between 92.4% and 94.9% of points in the acceptable plans passed the gamma analysis, while 86.8% to 98.3% of the points in the unacceptable plans passed the gamma analysis. The difference between acceptable and unacceptable plans was diminished further when gamma criteria were expanded to 3%/3 mm DTA. A fraction of pixels passing the gamma analysis was found to be a poor predictor of dosimetric accuracy with both planar dosimeters, as well as both sets of gamma criteria.Conclusions: Deconstruction of an IMRT plan for field-by-field QA requires complex analysis methods such as the gamma function. Distance to agreement, a component of the gamma function, has clinical relevance in a composite plan but when applied to individual, highly modulated fields, it can mask important dosimetric errors. While single field planar dosimetry may comprise one facet of an effective QA protocol, gamma analysis of single field measurements is insensitive to important dosimetric inaccuracies of the overall plan. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3425781]